Burners of the kind described usually have a cylindrical body into which a lance projects.
The lance is L-shaped and is usually provided with two or also more than two (i.e. three) coaxial ducts having nozzles at their ends.
During operation the nozzles must have substantially coaxial axes; this feature is of great importance, because through the inner duct a fuel (either liquid or gaseous fuel) passes through, to be injected through the nozzles, and through the outer duct air passes through, to be injected together with the fuel (this is the so called shielding air).
It is clear that in case the axes of the nozzles are not correctly aligned, fuel and air are not injected correctly and fuel penetration into the hot gases could be negatively affected.